The present invention relates to an antenna using an antenna element formed from a helical coil or the like, and a method for manufacturing such an antenna.
Antennas using antenna elements formed from helical coils etc. are generally used as ones for automobile use or for cellular phone use. In such an antenna, a change in pitch of a helical coil or the like results in a change in its antenna characteristic. In order to retain a constant antenna characteristic, the shape of the helical coil has to be fixed to prevent its pitch or the like from being changed.
Japanese Patent Publication No. 2000-252733A discloses an antenna element formed of a helical coil is first inserted into a mold, and the outer circumference of the antenna element is brought into contact with the inner surface of the mold so as to be placed at a predetermined position therein. Then, insulating resin is injected into the mold so as to mold a resin molded member integrally with the antenna element.
Here, a plurality of recess portions having a predetermined depth are formed in the inner surface of the mold in advance so that protrusions having a predetermined height are formed on the outer circumferential surface of the resin molded member integrally therewith. The pitch of the antenna element is fixed by the resin molded member, and the shape of the antenna element is prevented from being deformed. Thus, the antenna characteristic is stabilized. The outer circumference of the antenna element is exposed from the outer circumferential surface of the resin molded member.
Therefore, the resin molded member is inserted into another mold, and the tips of the predetermined-height protrusions provided in the outer circumferential surface of the resin molded member are brought into contact with the inner surface of the mold so as to be placed at a predetermined position therein. Then, insulating resin is injected into the mold so that a resin coating having a thickness equal to the height of the protrusions is molded integrally with the resin molded member so as to cover the resin molded member with the resin coating. The antenna element is covered with the resin coating so that the outside of the antenna element is electrically insulated, while the antenna element is prevented from being exposed to the atmosphere in order not to be rusted.
In the above manufacturing method, the resin is injected into the mold for forming the resin coating, at a center part of the antenna in a longitudinal (axial) direction thereof. The injecting direction is substantially perpendicular to the axial direction of the antenna. In a case where the injection pressure of the resin is too high, the lateral injection force strongly acts on the resin molded member so that the resin molded member is apt to be out of its proper position in the mold because the protrusions provided in the outer circumferential surface of the resin molded member cannot bear the injection force. As a result, the resin molded member is not coated with the resin coating by a predetermined thickness. On the contrary, in a case where the injection pressure of the resin is too low, the resin cannot reach every part of the mold satisfactorily. As a result, a sink is apt to be formed in the resin coating so that a large thickness variation is occurred in the resin coating.
In a case where the resin is injected into the mold from the tip end side of the antenna, there is a tendency that the injected resin does not flow uniformly over the outer circumference of the tip end portion of the resin molded member. Then, the resin on the side where most of resin flowed presses the tip end portion of the resin molded member to the opposite side. As a result, the resin molded member can not retain its proper position in the mold, so that a large thickness variation is occurred in the resin coating.